1. Field of the Invention:
The present invention relates to a zoom lens system for use in an image projecting apparatus with K uml/o/ hler illumination, and more particularly to a projecting lens system wherein a pupil position of a zoom lens system is made invariable in an operation of zooming so as to satisfy the conditions of K uml/o/ hler illumination without changing the position of a condensing lens system of K uml/o/ hler illumination system.
2. Description of the Prior Arts:
K uml/o/ hler illumination has frequently been used in a projecting optical system since it has conventionally given an image having a uniform illumination intensity. One example of such projecting optical system employing K uml/o/ hler illumination system is a system for projecting images of microfilms.
A microfilm image projecting optical system employing conventional K uml/o/ hler illumination system will be explained hereinbelow with reference to FIG. 1. Specifically, the projecting lens system is a zoom lens system comprising from the enlargement side, i.e., from a screen 8, an aperture stop 7, a first lens group 5 of a positive refractive power and a second lens group 6 of a negative refractive power. This lens system projects an image on a film 4 to the screen 8 with the magnification varied. In order to illuminate the film 4, luminous flux from a light source 1 is formed parallel by a first condensing lens unit 2. Then, the image of the light source 1 is formed at the position of the aperture stop 7 of the projecting zoom lens system by a second condensing lens unit 3. In this way, the filament image of the light source 1 is formed at the pupil position of the projecting lens system by transparently illuminating the film 4, with the result that the conditions of K uml/o/ hler illumination is satisfied as well as an the image having a uniform and effective illumination density is projected on the screen 8.
As described above, the projecting lens system is a zoom lens system. Therefore, the position of the aperture stop 7 is altered in an operation of zooming from the longest focal length side shown in FIG. 1A to the shortest focal length side shown in FIG. 1B, the result of which is that the conditions of K uml/o/ hler illumination are not fulfilled. Accordingly, this projecting optical system is constructed such that either the first or second condensing lens group 2 or 3, or alternatively the whole condensing lens group is moved to fulfill the conditions of K uml/o/ hler illumination.
Namely, in order to maintain the conditions of K uml/o/ hler illumination in the operation of zooming, the condensing lens system has to be manually adjusted by users or to be moved in a mechanically interlocking relation with the zoom lens system, causing trouble in operating the machine or entailing an increased cost of the machine.